Conductive paste for large greensheet screening including high thixotropic agent content

ABSTRACT

Deformation of vias and shorting between vias during screening of conductive paste onto greensheets wider than 185 mm is avoided by a concentration of thixotropic agent in the conductive paste formulation to a concentration which raises the thixotropic index of the paste to within 5% to 25% of the maximum achievable thixotropic index for the organic vehicle in the paste. For a paste containing molybdenum powder in which the organic vehicle is a mixture of ethyl cellulose, texanol and oleoyl sarcosine, this criterion corresponds to a concentration of glycerl tri(-12-hydroxystearate) in the range of 1.4% to 1.8% by weight.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/829,809filed Mar. 27, 1997, now U.S. Pat. No. 5,783,113, issued Jul. 21, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the manufacture ofmulti-layer ceramic (MLC) circuit packages and, more particularly, toscreening conductive patterns and via filling at small via spacing onwide greensheets.

2. Description of the Prior Art

It has been recognized for some years that miniaturization of electroniccircuits produces both manufacturing economies and performanceenhancement of the circuits. Small spacing between connected electronicelements results in reduced signal propagation time between circuitelements as well as allowing increased numbers of electronic elements tobe placed on a single chip and formed on a single wafer of particulardimensions with a particular series of process steps. Thus the cost ofthe process can be spread over an increased number of chips of increasedperformance as well as potentially increased functionality.

Some limits on integration density and circuit miniaturization areimposed by process incompatibility and conflicts for different types ofcircuit elements. For example, while hybrid BiCMOS devices are known andhave been successfully fabricated and used for some purposes, bipolarand CMOS processes are largely incompatible. Similarly, processes formaking large capacity dynamic random access memories includingpotentially millions of small capacitors on or in the substrate are verydifferent from the processes used to form and interconnect transistorsin logic arrays. While such conflicts could be approached throughmasking during chip formation, process complexity is multiplied andmanufacturing yield is compromised.

Accordingly, extremely complex electronic circuits have been formed atvery small size using multi-layer modular (MLM) packages and multi-layerceramic (MLC) packages in particular. These packages are formed byforming conductive patterns on insulating lamina with conductivematerials filling holes therein to form vias. While polyimide is oftenused for MLM packages, an uncured ceramic material referred to as agreensheet is used for MLC packages. (A greensheet is of a "rubbery"consistency in its uncured state when screening takes place.) The laminaare then stacked and laminated together to form a robust structure witha complex circuit embedded therein for interconnection of a potentiallylarge plurality of chips which may be formed by independent andpotentially incompatible processes. Connections between conductivepatterns on respective lamina and to mounting pads for the integratedcircuits and connection pins and the like on surfaces of the module aremade by the conductive material filling the vias.

It should be understood that the same economic and performanceincentives toward higher integration density on a chip, often incombination with somewhat increased chip area, also apply to suchmodular circuit packages. Therefore, modular circuit packages withincreased area and decreased size of vias and spacing between vias havebeen attempted. At the present state of the art, good manufacturingyields have been achieved with 185 mm wide greensheets having a viaspacing of 9 mils, via diameters of 4 mils and greensheet thicknesses of6 mils. However, acceptable manufacturing yields have not been achievedwith a 215 mm wide power plane greensheet having a larger 5.5 mil viadiameter for 8 and 11 mil thickness greensheets and a via pitch of 10mils or less. (The larger diameter vias provided to increasecurrent-carrying capability thus exceed one-half of the via pitch. Notealso that the increased lamina thickness either maintains or increasesvia aspect ratio.) Power planes form a significant portion of MLCpackages, particularly where different voltages must be supplied to aplurality of chips in the MLM or MLC package which may be formed bydifferent technologies and require different operating and signalvoltages.

On the larger greensheets, the screening paste which is used to fillvias tends to distort the greensheet at the bottom of the vias atpressures sufficient to achieve via filling, producing oversize vias.Oversize vias violate spacing rules and may even become shortedtogether. The reason for these distortions is not precisely known sinceit is presently impossible to observe the mechanics and relative motionof the greensheet and paste (between a backing sheet and a mask below anozzle through which the paste is extruded) during screening but, whilenot wishing to be held to any particular theory, is probably due, atleast in part; to a combination of factors including the greater elasticstrain which may occur in a larger greensheet for a given amount offorce applied thereto (thus inferring that some aspects of the presentprocessing of 185 mm greensheets have relatively small processtolerances).

Reduction in screening pressure or variation of other screening processparameters to avoid such distortions has uniformly resulted ininsufficient via filling for reliable connections to be made betweenlamina or failure to rectify the problem of oversize and/or shortedvias. As a result, virtually 100% of the wider greensheets included atleast one instance of insufficient via fill, spacing violations orshorting between vias and no combination of variation of processparameters and/or known paste formulation was capable of significantlyreducing the incidence of such malformations, much less produce aneconomically acceptable manufacturing yield.

Thixotropic additives are known in formulations of screening pastes forMLC package manufacture. For example, numerous thixotropic additives aredisclosed and discussed in U.S. Pat. No. 5,503,777, hereby fullyincorporated by reference. Essentially, a suitable paste will contain aconductive powder (and possibly non-conductive fillers) and an organicvehicle including an organic binder. While this paste will exhibit somethixotropy (e.g. the "thixotropic index" is the negative of the slope ofthe curve of viscosity as a function of shear rate), such a materialwill maintain a relatively constant viscosity with shearing forcesapplied thereto and the resulting shear rate.

A thixotropic agent is generally a metal organic ("organometallic")compound which, when added to the above paste at a low concentration,reacts with the organic vehicle. The reaction enhances a property knownas thixotropy such that viscosity decreases significantly withincreasing shear rate and vice-versa to increase the thixotropic indexof the paste. That is, if no force is applied thereto, a thixotropicpaste tends to behave as a solid but the material flows increasingly inthe manner of a liquid as increased force is applied. Viscosity willoften decrease below that of a non-thixotropic paste (e.g. a pastewithout a thixotropic agent even though it may exhibit some degree ofthixotropy) at high shear rates.

Thus, the above-incorporated U.S. patent employs a low concentration(0.5% of the paste while the organic vehicle is generally between orless than 15%-20% of the paste by weight which achieves a three-foldincrease in thixotropic index) of thixotropic agent sufficient toprevent flow of paste (referred to therein as "leveling") as the solventis removed after screening by further processing or absorption by thegreen sheet while leaving the screening process substantiallyunaffected. Leveling otherwise produces a dimple in the top of the viadue to reduction of paste volume. The above-incorporated U.S. patentalso reports a marked reduction in thixotropic index (e.g. reduction ineffectiveness of the thixotropic agent to increase the thixotropic indexand/or causing a decrease the thixotropic index) as the amount ofthixotropic agent is increased (as generally depicted in FIG. 5) from1.0% to 2.0% for tetrabenzyl orthosilicate and from 2.0% to 4.0% forsilicon 2-ethyl-hexanoate (merely doubling the thixotropic index at 0.5%at a 2.0% concentration).

Therefore it has been conventional to maintain the concentration ofthixotropic additives very low in order to decrease or avoid increase ineffective viscosity during screening so as to properly fill narrow orhigh aspect ratio vias and properly form fine conductive patterns on thegreensheets while presenting a viscosity sufficient to resist internalstresses during further processing after the screening process iscompleted when no further flow of paste is intended. Nevertheless, noknown paste formulation either with or without thixotropic agents hasbeen suitable for achieving even marginally acceptable manufacturingyields on 215 mm or larger power plane greensheets.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor increasing manufacturing yield in MLC circuit packages and powerplane greensheets larger than 185 mm suitable for use in modular circuitpackages.

It is another object of the invention to provide a conductive pasteformulation for screening which does not result in via distortion of MLMor MLC lamina or shorting between vias therein during screening.

In order to accomplish these and other objects of the invention, aconductive paste is provided including a conductive powder, an organicvehicle including an organic binder, and a thixotropic agent at aconcentration which raises the thixotropic index of the paste to within5%-25% of the maximum achievable thixotropic index for the paste,generally corresponding to a concentration in the range of 1.4%-1.8% byweight of the paste.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a plan view of a portion of a greensheet including deformedand shorted vias as produced with prior paste formulations,

FIG. 2 is a cross-section of a portion of FIG. 1 including two adjacentvias,

FIG. 3 is a plan view of a greensheet after screening with a paste inaccordance with the invention,

FIG. 4 is a cross-section of a portion of the greensheet of FIG. 3showing two adjacent vias, and

FIG. 5 graphically depicts the general behavior of changes in thixotropywith concentration of thixotropic agent for an arbitrary paste andthixotropic agent combination.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1 and 2,there is shown in magnified plan view and in further enlargedcross-section, respectively, a portion of the bottom of a 215 mmgreensheet 10 after screening with a conductive paste in accordance withprior methodologies and paste formulations. It is to be understood,however, that no portion of FIG. 1 or FIG. 2 is admitted to be prior artas to the present invention but, rather, is presented in the interest ofconveying a clear understanding of the meritorious effects of thepresent invention, as illustrated in FIGS. 3 and 4 as compared tomethods and materials over which the present invention constitutes animprovement.

As can be seen from FIGS. 1 and 2, while the majority of vias areproperly formed, vias 12 are formed oversize (O/S) and vias 14, 14' areformed shorted together. It may be noted that while the via diameter, asillustrated, is less than one-half the via pitch rather than exceedingone-half the via pitch in a power plane greensheet, as alluded to above,the via distortion and via shorting is still observed. The shorting ofvias 14, 14' clearly represents a defect in the via-filled greensheet.Oversize vias 12 may or may not cause a defect when the lamina isstacked with other lamina but are clearly a violation of spacing designrules and are therefore likely to cause a defect.

If a defect is, in fact, caused upon lamination with other lamina, it isimpossible to correct without the ability to form proper connectionselsewhere by a so-called engineering change (EC) which is costly interms of time to carry out the EC as well as the design and fabricationof redundant structures within the MLM or MLC in order to enable lateruse thereof for such ECs and the space consumed on the MLM or MLC by theredundant structures. The only other potential solution would be toincrease spacing between vias which carries the same expense of laminaspace and limits the degree of miniaturization potentially achievable.

As seen in FIG. 2, taken along section 2--2 of FIG. 1, upon filling, theperimeter of the vias 12 appears to be pressed outwardly, as indicatedat 16, against the resilient greensheet 10 by the pressure of the pasteduring the screening process. While the mechanism causing such viadistortion is not known, as alluded to above, the increased lateraldimensions of the greensheet together with sequential or unequal fillingof adjacent vias at any given point in time during the via fillingprocess may cause localized lateral elastic strain in the greensheet asindicated by arrows 20 which may result in a localized wiping action ofthe greensheet against the screening support S as screening nozzle 22traverses the screening mask 24 and green sheet 10. If present, thiseffect would aggravate the spreading of paste at the bottom of the viasand increase enlargement of the vias at the bottom of the greensheet 10.In a severe case, screening paste may then become trapped between thegreensheet 10 and the screening support S as shown by dashed line 18,resulting in shorting 14 of vias.

Referring now to FIGS. 3 and 4, it has been found by the inventors thatincreasing the concentration of thixotropic agent to between 1.4% to1.8% by weight and preferably about 1.6% by weight, allows reliable viafilling without distortion of the bottoms of the vias of the greensheeteven with increased lateral greensheet dimensions. This concentration ofthixotropic agent is much greater than would normally be used to preventpost-screening movement of paste, as disclosed in the above-incorporatedU.S. patent and, while the thixotropic index of the paste formulation isnot presently known, is believed to be within about 5%-25% of themaximum thixotropic index obtainable for the paste. This range ofconcentration of the paste formulation allowed a manufacturing yield offilled greensheets in excess of 95% defect-free in comparison with anear zero percent defect-free yield with any other formulation orscreening process modification or variation.

For comparison, a 1.0% concentration, by weight, of thixotropic agent inthe paste produced good manufacturing yield during screening and viafilling of 185 mm wide but was completely unsuccessful in screening andfilling the wider 215 mm greensheets without producing via distortionand/or shorting between vias. While determination of thixotropic indexis somewhat uncertain (being a "best fit" of a line through empiricallydetermined values of viscosity and shear rate; both such measurementsbeing subject to substantial uncertainty and error), the 1.6%thixotropic agent formulation appears to represent only a 5% increase inthixotropic index over that of the 1.0% thixotropic agent formulation(although the thixotropic index of the 1.0% thixotropic agentformulation is already relatively high). When it is observed that thedifference in width between the 185 mm and 215 mm greensheets is onlyabout 10% while diminishing the manufacturing yield to zero and that amanufacturing yield in excess of 95 % can be developed only by theincrease in thixotropic index to within 5% to 25% of maximum for a givenpaste (even when it represents only a seemingly slight increase over acompletely unsuccessful formulation), it is seen that the invention ishighly counter-intuitive and provides a solution to a problem having apreviously unrecognized and severe criticality.

The invention is preferably practiced using a moly paste comprisingmolybdenum powder, and an organic vehicle including ethyl cellulose,TEXANOL (an ester alcohol), and oleoyl sarcosine with a 1.6%concentration by weight of a commercially available thixotropic agentknown in the art as thixcin (CAS registry number 9001-78-3) which isalso known as hydrogenated castor oil or castor wax. The chemical namefor the material is glyceryl tri(12)-hydroxystearate. The preferredmaterial is commercially available under the name of "THIXCIN R" whichis a trademark of NL Industries, Inc. However, the principles of theinvention are applicable to any other thixotropic agent which to reactspredictably with any particular known screening paste formulation toincrease thixotropic index, such as those alluded to above disclosed inthe above-incorporated U.S. patent, as will be apparent to those skilledin the art in view of this disclosure. The meritorious effects of theinvention will be produced as long as the viscosity of the thixotropicpaste formulation can reach viscosities equal to or less than theviscosity of the paste without the thixotropic agent at practicalscreening pressures and shear rates resulting therefrom and whichotherwise allow filling of the vias in green sheets.

The addition of a thixotropic agent to a screening paste at greater thannormal concentration (e.g. to suppress undesired paste movement duringpost-screening processing) is effective to provide extremely highmanufacturing yield and nearly complete avoidance of via distortion andshorting during filling of greensheets having lateral dimensions of 215mm or greater where defect-free screening could not previously beaccomplished. It is possible, in this regard, that the effects of theinvention may be enhanced by increased shear rate (SR of FIG. 4) due tohigh via aspect ratio or diminished diameter of vias and may allow areduction in via spacing or pitch or even increased paste pressures forfilling fine vias. As shown by the dashed line graphically representingthe projected paste pressure P of FIG. 4 within a via as via filling iscompleted, the thixotropic viscosity of the paste in accordance with theinvention is believed to cause a pressure drop or gradient over the viaheight due to increasing levels of viscous drag as paste movement andshear rate becomes vanishingly small as the paste motion ceases withinthe vias.

In view of the foregoing, it is clearly seen that the invention providesa large and unexpected improvement in manufacturing yield of filledgreensheets of dimensions in excess of 185 mm by utilization of acounter-intuitive screening paste formulation having an increasedconcentration of a thixotropic agent several times greater than in knownformulations. The screening paste formulation with a high concentrationof a thixotropic agent in accordance with the invention avoids virtuallyall via distortion during screening which may be significant tomanufacturing yield while providing reliable and consistent via fillingwithout modification of any other screening process parameters oradditional process steps.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described my invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A conductive paste,comprising:a conductive powder, an organic vehicle comprising an organicbinder, and a thixotropic agent comprising hydrogenated castor oil, saidhydrogenated castor oil contained at a concentration effective toprovide a viscosity in said conductive paste that is equal to or lessthan the viscosity of said conductive paste where devoid of saidthixotropic agent, and at screening pressures allowing filling of viasin a 215 mm wide green sheet having 5.5 mil via diameter, a via pitchless than 10 mils and a thickness of 8 to 11 mils.
 2. A conductive pasteas recited in claim 1, wherein said conductive powder is a molybdenumpowder.
 3. A conductive paste as recited in claim 1, wherein saidorganic vehicle is a mixture comprised of ethyl cellulose, ester alcoholand oleoyl sacosine.
 4. A conductive paste as recited in claim 3,wherein said thixotropic agent is glyceryl tri(12)-hydroxystearate.
 5. Aconductive paste, comprising:a conductive powder, an organic vehiclecomprising an organic binder, and a thixotropic agent comprisinghydrogenated castor oil, said hydrogenated castor oil contained at aconcentration in said paste ranging from 1.4 to 1.8% by weight,effective to provide a viscosity in said conductive paste that is equalto or less than the viscosity of said conductive paste where devoid ofsaid thixotropic agent, and at screening pressures allowing filling ofvias in a 215 mm wide green sheet having 5.5 mil via diameter, a viapitch less than 10 mils and a thickness of 8 to 11 mils.
 6. A conductivepaste, comprising:a conductive powder, an organic vehicle comprising anorganic binder, and a thixotropic agent comprising hydrogenated castoroil, said hydrogenated castor oil contained at a concentration in saidpaste is 1.6% by weight, effective to provide a viscosity in saidconductive paste that is equal to or less than the viscosity of saidconductive paste where devoid of said thixotropic agent, and atscreening pressures allowing filling of vias in a 215 mm wide greensheet having 5.5 mil via diameter, a via pitch less than 10 mils and athickness of 8 to 11 mils.